# Charging car battery with another car - circuit schematic

I came across an example in a textbook that talked about charging a dead car battery with another car battery. They provided this picture of how to connect the two batteries: -

Source: Engineering Circuit Analysis - 12th edition - page 6 - Irwin

They correctly note that because the current flows from + to - in the "Weak battery" it charges, while the "Good battery" discharges, because the current flows from - to +.

Let's assume the "Good battery" is 12 V and the "Weak battery" is 9 V. Then, the associated circuit diagram must look like this: -

simulate this circuit – Schematic created using CircuitLab

The diagram contains a contradiction, because the voltage drop cannot be both 12 V and 9 V. But in real life, it is indeed possible to charge a dead car battery like this. What is missing from the circuit diagram for this to not contradict itself?

• The circuit diagram is always a simplification of reality. "All models are wrong, but some are useful." Here you have found one of the situations where the simplification is not quite good enough and you have to make it a bit more complex if you want to use this diagram to calculate things. Commented Aug 30, 2023 at 16:27